This invention relates to a combustion system and method, and, more particularly, to such a system and method in which a plurality of adjacent and opposing enclosures including furnace sections and recycle sections are provided for receiving fluidized beds.
Fluidized bed combustion systems are well known and include a furnace section in which air is passed through a bed of particulate material, including a fossil fuel, such as coal, and a sorbent for the oxides of sulfur generated as a result of combustion of the coal, to fluidize the bed and to promote the combustion of the fuel at a relatively low temperature. These types of combustion systems are often used in steam generators in which water is passed in a heat exchange relationship to the fluidized bed to generate steam and permit high combustion efficiency and fuel flexibility, high sulfur adsorption and low nitrogen oxides emissions.
A typical fluidized bed utilized in the furnace section of these type systems is commonly referred to as a "bubbling" fluidized bed in which the bed of particulate material has a relatively high density and a well defined, or discrete, upper surface. Other types of systems utilize a "circulating" fluidized bed in which the fluidized bed density is below that of a typical bubbling fluidized bed, the fluidizing air velocity is equal to or greater than that of a bubbling bed, and the flue gases passing through the bed entrain a substantial amount of the fine particulate solids to the extent that they are substantially saturated therewith.
Circulating fluidized beds are characterized by relatively high internal and external solids recycling which makes them insensitive to fuel heat release patterns, thus minimizing temperature variations and, therefore, stabilizing the sulfur emissions at a low level. The external solids recycling is achieved by disposing a cyclone separator at the furnace section outlet to receive the flue gases, and the solids entrained thereby, from the fluidized bed. The solids are separated from the flue gases in the separator and the flue gases are passed to a heat recovery area while the solids are recycled back to the furnace. This recycling improves the efficiency of the separator, and the resulting increase in the efficient use of sulfur adsorbent and fuel residence time reduces the adsorbent and fuel consumption. U.S. Pat. Nos. 5,040,492 and 5,054,436, assigned to the same assignee as the present application, disclose systems in which the separated solids are recycled back to the furnace.
U.S. Pat. Nos. 4,609,623 and 4,809,625, assigned to the same assignee as the present application, disclose a fluidized bed reactor in which a dense, or bubbling, bed is maintained in the lower portion of the furnace, while the bed is otherwise operated as a circulating bed. This hybrid arrangement results in several advantages not the least significant of which is the ability to utilize fuel and adsorbent over a relatively large particle size range.
In designing fluidized bed combustion systems of the above types, increases in furnace capacity from a given design are usually achieved by increasing the height of the furnace walls. However, this is expensive and there are certain limits to the height of the walls. It has therefore been suggested that the size of the furnace, and therefore its capacity, be increased by increasing the size of the furnace in "plan view" i.e., increasing the width and/or the depth of the furnace. However, this usually requires a common wall, or the like, to be placed in the furnace section to divide the area into two or more fluidized beds which requires separate operating controls, etc. which is expensive. Also, the common wall is subjected to lateral loading, especially when the multiple beds operate differently or if one bed is rendered inoperable due to equipment failure. This lateral loading can cause damage to the wall and attendant reduction i operation and efficiency.
Increases in furnace capacity also lead to the use of larger cyclone separators which permit increasing amounts of fine, unburnt fuel particles to escape with the separated flue gases. This escape of unburnt fuel particles reduces fuel efficiency, thereby increasing fuel consumption.